Resistive Switching Layers Including Hf-Al-O

ABSTRACT

Provided are resistive random access memory (ReRAM) cells having switching layers that include hafnium, aluminum, oxygen, and nitrogen. The composition of such layers is designed to achieve desirable performance characteristics, such as low current leakage as well as low and consistent switching currents. In some embodiments, the concentration of nitrogen in a switching layer is between about 1 and 20 atomic percent or, more specifically, between about 2 and 5 atomic percent. Addition of nitrogen helps to control concentration and distribution of defects in the switching layer. Also, nitrogen as well as a combination of two metals helps with maintaining this layer in an amorphous state. Excessive amounts of nitrogen reduce defects in the layer such that switching characteristics may be completely lost. The switching layer may be deposited using various techniques, such as sputtering or atomic layer deposition (ALD).

TECHNICAL FIELD

The present invention relates generally to non-volatile memory devicesand more specifically to resistive switching layers including hafnium,aluminum, nitrogen, and oxygen and related to resistive random accessmemory (ReRAM) cells including such layers.

BACKGROUND

Nonvolatile memory is computer memory capable of retaining the storedinformation even when unpowered. Non-volatile memory is typically usedfor the task of secondary storage or long-term persistent storage andmay be used in addition to volatile memory, which losses the storedinformation when unpowered. Nonvolatile memory can be permanentlyintegrated into computer systems (e.g., solid state hard drives) or cantake the form of removable and easily transportable memory cards (e.g.,USB flash drives). Nonvolatile memory is becoming more popular becauseof its small size/high density, low power consumption, fast read andwrite rates, retention, and other characteristics.

Flash memory is a common type of nonvolatile memory because of its highdensity and low fabrication costs. Flash memory is a transistor-basedmemory device that uses multiple gates per transistor and quantumtunneling for storing the information on its memory device. Flash memoryuses a block-access architecture that can result in long access, erase,and write times. Flash memory also suffers from low endurance, highpower consumption, and scaling limitations.

The constantly increasing speed of electronic devices and storage demanddrive new requirements for nonvolatile memory. For example, nonvolatilememory is expected to replace hard drives in many new computer systems.However, transistor-based flash memory is often inadequate to meet therequirements for nonvolatile memory. New types of memory, such asresistive random access memory, are being developed to meet thesedemands and requirements.

SUMMARY

Provided are resistive random access memory (ReRAM) cells havingswitching layers that include hafnium, aluminum, oxygen, and nitrogen.The composition of such layers is designed to achieve desirableperformance characteristics, such as low current leakage as well as lowand consistent switching currents. In some embodiments, theconcentration of nitrogen in a switching layer is between about 1 and 20atomic percent or, more specifically, between about 2 and 5 atomicpercent. Addition of nitrogen helps to control concentration anddistribution of defects in the switching layer. Also, nitrogen as wellas a combination of two metals helps with maintaining this layer in anamorphous state. Excessive amounts of nitrogen reduce defects in thelayer such that switching characteristics may be completely lost. Theswitching layer may be deposited using various techniques, such assputtering and atomic layer deposition (ALD).

In some embodiments, a resistive random access memory cell includes afirst layer operable as a first electrode, a second layer operable as asecond electrode, and a third layer operable as a resistive switchinglayer and disposed between the first layer and the second layer. Thethird layer includes hafnium, aluminum, oxygen, and nitrogen. Aconcentration of oxygen in the third layer may be between about 30 and60 atomic percent. In some embodiments, nitrogen's concentration in thethird layer is between about 1 and 20 atomic percent. The concentrationof nitrogen in the third layer may be at least three times less than aconcentration of oxygen. The concentration of hafnium in the third layermay at least twice greater than a concentration of aluminum. Theconcentration of hafnium in the third layer may be between about 5 and40 atomic percent. The concentration of aluminum in the third layer isbetween 3 and 20 atomic percent.

In some embodiments, the thickness of the third layer is between about20 and 100 Angstroms. The third layer may be substantially amorphous. Insome embodiments, nitrogen is unevenly distributed within the thirdlayer. The third layer may include a sublayer at the interface with thefirst layer such that a thickness of the sublayer is one tenth of athickness of the third layer. In this case, at least about 50 atomicpercent of the nitrogen in the third layer is provided within thesublayer.

In some embodiments, the first layer is formed after the third layer.The first layer may directly interface the third layer. The first layermay include one of tantalum nitride, titanium nitride, or tungstennitride. The first layer or the second layer may have a thickness ofless than 50 Angstroms.

Provided also is a method of forming a resistive random access memorycell. The method may involve providing a substrate including a firstelectrode, depositing a stack of layers over the first electrode usingatomic layer deposition (ALD), and incorporating nitrogen into thestack. Incorporation of nitrogen into the stack forms a resistiveswitching layer. The stack may include hafnium oxide and aluminum oxide.

In some embodiments, depositing the stack involves repeating adeposition cycle until reaching a predetermined thickness of the stack.Each deposition cycle involves depositing multiple hafnium oxide layersand depositing one aluminum oxide layer. A ratio of hafnium oxide layersto aluminum oxide layers in each cycle may be at least about five.Depositing the stack may involve repeating a deposition cycle untilreaching a predetermined thickness of the stack such that eachdeposition cycle involves forming a saturated layer using a hafniumcontaining precursor and an aluminum containing precursor and oxidizingthe saturated layer. Forming the saturated layer may involve exposing adeposition surface to the hafnium containing precursor followed byexposing the deposition surface including the hafnium containingprecursor to the aluminum containing precursor.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate understanding, the same reference numerals have been used,where possible, to designate common components presented in the figures.The drawings are not to scale and the relative dimensions of variouselements in the drawings are depicted schematically and not necessarilyto scale. Various embodiments can readily be understood by consideringthe following detailed description in conjunction with the accompanyingdrawings, in which:

FIGS. 1A and 1B illustrate schematic representations of a ReRAM cell inits high resistive state (HRS) and low resistive state (LRS), inaccordance with some embodiments.

FIG. 2 illustrates a plot of a current passing through a ReRAM cell as afunction of a voltage applied to the ReRAM cell, in accordance with someembodiments.

FIG. 3A illustrates a process flowchart corresponding to a method offorming a ReRAM cell, in accordance with some embodiments.

FIG. 3B further illustrates various features of the base layer formationoperation, in accordance with certain embodiments.

FIG. 4 illustrates a schematic representation of a ReRAM cell, inaccordance with some embodiments.

FIG. 5 illustrates a schematic representation of an atomic layerdeposition apparatus for fabricating ReRAM cells, in accordance withsome embodiments.

FIGS. 6A and 6B illustrate schematic views of memory arrays includingmultiple ReRAM cells, in accordance with some embodiments.

DETAILED DESCRIPTION

A detailed description of various embodiments is provided below alongwith accompanying figures. The detailed description is provided inconnection with such embodiments, but is not limited to any particularexample. The scope is limited only by the claims and numerousalternatives, modifications, and equivalents are encompassed. Numerousspecific details are set forth in the following description in order toprovide a thorough understanding. These details are provided for thepurpose of example and the described techniques may be practicedaccording to the claims without some or all of these specific details.For the purpose of clarity, technical material that is known in thetechnical fields related to the embodiments has not been described indetail to avoid unnecessarily obscuring the description.

INTRODUCTION

A ReRAM cell exhibiting resistive switching characteristics generallyincludes multiple layers formed into a stack. This stack is sometimesreferred to as a Metal-Insulator-Metal (MIM) stack. The stack includestwo conductive layers operating as electrodes. These conductive layersare identified as “M” and may include metals and other conductivematerials, such as doped silicon. The stack also includes an insulatorlayer provided in between the electrode and is indentified as “I”. Theinsulator layer exhibits changes in its resistive propertiescharacterized by applying predetermined voltages. These resistive statesmay be used to represent one or more bits of information. As such, theinsulator layer is often referred to as a resistive switching layer andthe overall ReRAM cell may be referred to as resistive random accessmemory (ReRAM) cell.

It has been found that certain materials exhibit resistive switchingcharacteristics and, thus, may be suitable materials for resistiveswitching layers in ReRAM cells. Generally, these materials have someforms of defects that allow them to switch. For example, metal oxidesmay be formed with some oxygen vacancies that increase conductivity ofthese oxides. The oxygen vacancies may be created by oxygen removal fromstoichiometric oxides, doping, and other techniques further describedbelow. These defects are believed to form conductive paths or filamentsduring initial activation of the resistive switching layers by applyinga forming voltage. The filaments can be broken and reformed duringswitching to provide different resistive stated of the ReRAM cellsthereby allowing to store data in these cells.

Practical applications of ReRAM cells require certain switching, dataretention, and other characteristics. For example, ReRAM cells need tohave low leakage, low switching currents, stable performance over alarge number of switching cycles. More specific examples of thesecharacteristics are described below with reference to FIGS. 1A-1B and 2.It has been found that a composition and morphology of resistiveswitching layers has to be specifically tuned to meet theserequirements. For example, hafnium and aluminum oxides may have poorswitching characteristics when used independently, i.e., a switchinglayer including non-stoichiometric hafnium oxide or a switching layerincluding non-stoichiometric aluminum oxide. Combining hafnium andaluminum oxides in the same resistive switching layer improves some ofthe switching characteristics. Without being restricted to anyparticular theory, it is believed that a combination of hafnium andaluminum oxides may help to improve these switching characteristics(e.g., by distorting lattices and/or forming oxygen vacancies).

It has been found that some switching characteristics can be furtherimproved by introducing nitrogen into oxides (or introducing oxygen intonitrides). This switching behavior was unexpected. For example, additionof nitrogen helps to maintain the resistive switching layers in anamorphous state thereby improving their long term performance.Furthermore, addition of some nitrogen reduces the set and resetcurrents through the layers, which in turn reduces power consumption,reduces heating, and also improves long term performancecharacteristics. Without being restricted to any particular theory, itis believed that addition of nitrogen effectively controls the amount ofoxygen vacancies and fewer conductive filaments are formed as a resultof this addition. However, switching layers with excessive amounts ofnitrogen have shown poor resistive switching characteristics. It isbelieved that substantial reduction of oxygen vacancies reduces theability of these films to form conductive filaments.

Provided are ReRAM cells having switching layers that include hafnium,aluminum, oxygen, and nitrogen. This combination of four elements is notlimited to any particular stoichiometric relationship or formula. Thecomposition of switching layers is specifically designed to achievedesirable performance characteristics of ReRAM cells. In someembodiments, a switching voltage is less than about 2 V, while thecorresponding switching current is less than about 100 μA, whichprovides improved (lower) power characteristics.

In some embodiments, the concentration of nitrogen in the switchinglayer is between about 1 and 20 atomic percent or, more specifically,between about 2 and 5 atomic percent. The nitrogen concentration may beat least three times less than the oxygen concentration in the samelayer. In some embodiments, the concentration of oxygen in the resistiveswitching layer is between about 30 and 60 atomic percent or, morespecifically between about 40 and 50 atomic percent. As stated above,addition of nitrogen may be used for controlling the amount of oxygenvacancies in the layer and for maintaining this layer in an amorphousstate. Furthermore, addition of aluminum into the layer also helps toreduce its crystallization during annealing and/or switching.

In some embodiments, the concentration of hafnium in the resistiveswitching layer is between about 5 and 40 atomic percent or, morespecifically, between about 10 and 30 atomic percent. The concentrationof aluminum in the same layer may be between 3 and 20 atomic percent or,more specifically, between about 5 and 10 atomic percent. In someembodiments, the hafnium concentration is at least twice greater thanthe aluminum concentration.

The fabrication process may include formation of a base layer, which maybe deficient in oxygen and/or nitrogen. The deficient element is addedto convert the base layer into a resistive switching layer. For example,a base layer may include hafnium and aluminum oxides and substantiallyno nitrogen. Nitrogen is then added during a later nitriding operation.In a similar manner, a base layer may include hafnium and aluminumnitrides and substantially no or an insufficient amount of oxygen.Oxygen is then added during a later oxygenation operation.Alternatively, the base layer may be formed with all necessarycomponents and no separate nitriding or oxygenation operations may beneeded. In this case, the base layer is formed already as a resistiveswitching layer.

A base layer may be deposited using various techniques, such assputtering and atomic layer deposition (ALD). The ALD approaches may befurther divided into nanolamination ALD or staggered pulse ALD. Thefollowing description of these approaches is directed to initialformation of metal oxides followed by nitriding operations. However, onehaving ordinary skills in the art would understand that such processesmay also involve initial formation of metal nitrides followed byoxygenation operations.

Nanolamination may involve deposition of one or more hafnium oxidelayers and, separately, one or more aluminum oxide layers to form astack. The stack is then annealed in order to intermix these layers. Insome embodiments, the resulting layer (i.e., after the annealing) issubstantially homogeneous. For example, a hafnium containing precursor,such as TDMAHf, TEMAHf, and/or hafnium tetrachloride may be introducedinto an ALD chamber followed by an oxidizer, such as water or ozone. Ahafnium oxide layer may be formed at this point. This hafniumprecursor—oxidizer cycle may be repeated multiple times depending on adesired concentration of hafnium in a resulting base layer (andsubsequently in the resistive switching layer). An aluminum containingprecursor, such as a TMA, is then introduced into a chamber, followed byintroduction of an oxidizer, such as water or ozone. As such, a separatelayer of aluminum oxide is formed over one or more layers of hafniumoxide. Of course, this deposition order may be reversed, and one or morealuminum oxide layers may be deposited prior to deposition of one ormore hafnium oxide layers. Furthermore, deposition of one or morealuminum oxides layers and one or more hafnium oxide layers may berepeated a number of times until the base layer becomes sufficientlythick. The composition of the base layer can be modified by controllinghow many aluminum oxides layers and how many hafnium oxides layers aredeposited and form the base layer. For example, a base layer may beformed by alternating depositions of five hafnium oxide layers per onealuminum oxide layer. Furthermore, distribution of hafnium, aluminum,oxygen, and nitrogen in the resulting resistive switching layer may becontrolled by specific order of ALD cycles.

A staggered pulse deposition may involve introducing a hafniumcontaining precursor into an ALD chamber followed by introducing analuminum containing precursor into the ALD chamber. It should be notedthat both metal precursors are introduced into the chamber prior to anyoxidation of any one of the two precursors. In some embodiments, analuminum containing precursor is introduced into the chamber prior tointroducing a hafnium containing precursor. This order depends on thetype of metal precursors (their adsorption characteristics, size,reactivity, and other like characteristics) and desired composition ofthe resulting base layer. For example, if TMA is introduced into thechamber prior to TDMAHf, then the resulting base layer containspredominantly aluminum oxide with very little hafnium present. However,if TMAHf is introduced into the chamber prior to TDMA, then theresulting base layer contains about 30 atomic percent of aluminum oxideand 70 atomic percent of hafnium oxide relative to the total amount ofmetals present in the layer. In some embodiments, concentration ofhafnium relative to the total amount of metal (i.e., Hf/(Hf+Al)) rangesfrom 40 to 70 atomic percent.

Once both metal precursors are allowed to adsorb on the surface, anoxidizer is introduced into the chamber to convert both precursors intocorresponding oxides or nitrides. As such, a film containing bothaluminum and hafnium oxides (or nitrides) may be formed. This cycleincluding introduction of two metal precursors followed by introductionof an oxidizer may be repeated a number of times to build a base layerhaving a desired thickness. The base layer composition generally dependson the nature of precursors used in this approach.

Once the base layer is formed, the process may continue with introducingnitrogen (or oxygen) into this layer to convert the layer into aresistive switching layer. Various nitridation techniques may be used,such as plasma nitridation (e.g., de-coupled plasma nitridation) andthermal nitridation treatment (e.g., anneal). In some embodiments, someor all of the nitrogen may be introduced into the base layer during itsformation. For example, ammonia may be used an as a reactant during ALDprocessing to convert one or more metal containing precursors intonitride layers that are provided in the same stack with the oxide layer.If all nitrogen is introduced during formation of the base layer, thenno separate nitridation step is needed after formation of the baselayer. In other words, the base layer is already formed as a resistiveswitching layer.

Examples of ReRAM Cells and their Switching Mechanisms

A brief description of ReRAM cells is provided for better understandingof various features of resistive switching layers described in thisdocument. A ReRAM cell includes a resistive switching layer formed froma dielectric material exhibiting resistive switching characteristics. Adielectric, which is normally insulating, can be made to conduct throughone or more filaments or conduction paths formed after application of asufficiently high voltage. The conduction path formation can arise fromdifferent mechanisms, including defects, metal migration, and othermechanisms further described below. Once the one or more filaments orconduction paths are formed in the dielectric component of a memorydevice, these filaments or conduction paths may be reset (or brokenresulting in a high resistance) or set (or re-formed resulting in alower resistance) by applying certain voltages.

FIG. 1A illustrates a schematic representation of ReRAM cell 100including top electrode 102, bottom electrode 106, and resistiveswitching layer 104 provided in between top electrode 102 and bottomelectrode 106. It should be noted that the “top” and “bottom” referencesfor electrodes 102 and 106 are used solely for differentiation and notto imply any particular spatial orientation of these electrodes. Oftenother references, such as “first formed” and “second formed” electrodesor simply “first” and “second”, may be used identify and distinguish thetwo electrodes. ReRAM cell 100 may also include other components, suchas current limiting layers, diodes, and other components.

Resistive switching layer 104 may be initially formed from a dielectricmaterial. It later can be made to conduct through one or more filamentsor conduction paths formed by applying first a forming voltage (afterinitial fabrication) and later a set voltage (during operation). Toprovide this resistive switching functionality, resistive switchinglayer 104 includes a concentration of electrically active defects 108,which are sometimes referred to as traps. For example, some chargecarriers may be absent from the structure (i.e., vacancies) and/oradditional charge carriers may be present (i.e., interstitials)representing defects 108. In some embodiments, defects may be formed byimpurities (i.e., substitutions). These defects may be utilized forReRAM cells operating according to a valence change mechanism, which mayoccur in specific transition metal oxides and is triggered by amigration of anions, such as oxygen anions. Migrations of oxygen anionsmay be represented by the motion of the corresponding vacancies, i.e.,oxygen vacancies. A subsequent change of the stoichiometry in thetransition metal oxides leads to a redox reaction expressed by a valencechange of the cation sublattice and a change in the electricalconductivity. In this example, the polarity of the pulse used to performthis change determines the direction of the change, i.e., reduction oroxidation. Other resistive switching mechanisms include bipolarelectrochemical metallization mechanisms and thermochemical mechanisms,which leads to a change of the stoichiometry due to a current-inducedincrease of the temperature.

Without being restricted to any particular theory, it is believed thatdefects 108 can be reoriented within resistive switching layer 104 toform filaments or conduction paths as, for example, schematically shownin FIG. 1B as element 110. This reorientation of defects 108 occurs whena certain voltage is applied to electrodes 102 and 106. Sometimes,reorientation of defects 108 is referred to as “filling the traps” whena set voltage is applied (and forming one or more filaments orconduction paths) and “emptying the traps” when a reset voltage isapplied (and breaking the previously formed filaments or conductionpaths).

Defects 108 can be introduced into resistive switching layer 104 duringor after its fabrication. For example, a concentration of oxygendeficiencies can be introduced into metal oxides during their depositionor during subsequent annealing.

Operation of ReRAM cell 100 will now be briefly described with referenceto FIG. 2 illustrating a logarithmic plot of a current passing through aunipolar ReRAM cell as a function of a voltage applied to the electrodeof ReRAM cell, in accordance with some embodiments. Similarcharacteristics are demonstrated by unipolar cells, additional detailsof which are further presented below. ReRAM cell 100 may be either in alow resistive state (LRS) defined by line 124 or high resistive state(HRS) defined by line 122. Each of these states is used to represent adifferent logic state, e.g., HRS representing logic one and LRSrepresenting logic zero or vice versa. Therefore, each ReRAM cell thathas two resistive states may be used to store one bit of data. It shouldbe noted that some ReRAM cells may have three and even more resistivestates allowing multi-bit storage in the same cell.

HRS and LRS are defined by presence or absence of one or more filamentsor conductive paths in resistive switching layer 104 and formingconnections between these filaments or conduction paths and twoelectrodes 102 and 106. For example, a ReRAM cell may be initiallyfabricated in LRS and then switched to HRS. A ReRAM cell may be switchedback and forth between LRS and HRS many times, defined by set and resetcycles. Furthermore, a ReRAM cell may maintain its LRS or HRS for asubstantial period of time and withstand a number of read cycles.

The overall operation of ReRAM cell 100 may be divided into a readoperation, set operation (i.e., turning the cell “ON”), and resetoperation (i.e., turning the cell “OFF”). During the read operation, thestate of ReRAM cell 100 or, more specifically, the resistive ofresistive switching layer 104 can be sensed by applying a sensingvoltage to electrodes 102 and 106. The sensing voltage is sometimesreferred to as a “READ” voltage and indicated as V_(READ) in FIG. 2. IfReRAM cell 100 is in HRS represented by line 122, the external read andwrite circuitry connected to electrodes 102 and 106 will sense theresulting “OFF” current (I_(OFF)) that flows through ReRAM cell 100. Asstated above, this read operation may be performed multiple timeswithout switching ReRAM cell 100 between HRS and LRS. In the aboveexample, the ReRAM cell 100 should continue to output the “OFF” current(I_(OFF)) when the read voltage (V_(READ)) is applied to the electrodes.

Continuing with the above example, when it is desired to switch ReRAMcell 100 into a different logic state (corresponding to LRS), ReRAM cell100 is switched from its HRS to LRS. This operation is referred to as aset operation. This may be accomplished by using the same read and writecircuitry to apply a set voltage (V_(SET)) to electrodes 102 and 106.Applying the set voltage (V_(SET)) forms one or more filaments orconduction paths in resistive switching layer 104 and switches ReRAMcell 100 from its HRS to LRS as indicated by dashed line 126. It shouldbe noted that formation or breaking of filaments or conduction paths inresistive switching layer 104 may also involve forming or breakingelectrical connections between these filaments and one (e.g., reactiveelectrode) or both electrodes. The overarching concern is passage of thecurrent between the two electrodes.

In LRS, the resistive characteristics of ReRAM cell 100 are representedby line 124. In this LRS, when the read voltage (V_(READ)) is applied toelectrodes 102 and 106, the external read and write circuitry will sensethe resulting “ON” current (I_(ON)) that flows through ReRAM cell 100.Again, this read operation may be performed multiple times withoutswitching ReRAM cell 100 between LRS and HRS.

It may be desirable to switch ReRAM cell 100 into a different logicstate again by switching ReRAM cell 100 from its LRS to HRS. Thisoperation is referred to as a reset operation and should bedistinguished from set operation during which ReRAM cell 100 is switchedfrom its HRS to LRS. During the reset operation, a reset voltage(V_(RESET)) is applied to ReRAM cell 100 to break the previously formedfilaments or conduction paths in resistive switching layer 104 andswitches ReRAM cell 100 from its LRS to HRS as indicated by dashed line128. Reading of ReRAM cell 100 in its HRS is described above. Overall,ReRAM cell 100 may be switched back and forth between its LRS and HRSmany times. Read operations may be performed in each of these states(between the switching operations) one or more times or not performed atall. It should be noted that application of set and reset voltages tochange resistive states of the ReRAM cell involves complex mechanismsthat are believed to involve localized resistive heating as well asmobility of defects impacted by both temperature and applied potential.

ReRAM cell 100 may be configured to have either unipolar switching orbipolar switching. The unipolar switching does not depend on thepolarity of the set voltage (V_(SET)) and reset voltage (V_(RESET))applied to the electrodes 102 and 106 and, as a result, to resistiveswitching layer 104. In the bipolar switching the set voltage (V_(SET))and reset voltage (V_(RESET)) applied to resistive switching layer 104need to have different polarities.

In some embodiments, the set voltage (V_(SET)) is between about 100 mVand 10V or, more specifically, between about 500 mV and 5V. The lengthof set voltage pulses (t_(SET)) may be less than about 100 millisecondsor, more specifically, less than about 5 milliseconds and even less thanabout 100 nanoseconds. The read voltage (V_(READ)) may be between about0.1 and 0.5 of the write voltage (V_(SET)). In some embodiments, theread currents (I_(ON) and I_(OFF)) are greater than about 1 mA or, morespecifically, is greater than about 5 mA to allow for a fast detectionof the state by reasonably small sense amplifiers. The length of readvoltage pulse (t_(READ)) may be comparable to the length of thecorresponding set voltage pulse (t_(SET)) or may be shorter than thewrite voltage pulse (t_(RESET)).

A ratio of set and reset currents (i.e., a high I_(SET)/I_(RESET) ratio)that correspond to set voltage (V_(SET)) and reset voltage (V_(RESET))may be at least about 5 or, more specifically, at least about 10 to makethe state of ReRAM cell easier to determine. ReRAM cells should be ableto cycle between LRS and HRS between at least about 10³ times or, morespecifically, at least about 10⁷ times without failure. A data retentiontime (t_(RET)) should be at least about 5 years or, more specifically,at least about 10 years at a thermal stress up to 85° C. and smallelectrical stress, such as a constant application of the read voltage(V_(READ)). Other considerations may include low current leakage, suchas less than about 40 A/cm² measured at 0.5 V per 20 Å of oxidethickness in HRS.

In some embodiments, the same ReRAM cell may include two or moreresistive switching layers interconnected in series. Adjacent resistiveswitching layers may directly interface each other or be separated by anintermediate layer.

In some embodiments, a ReRAM cell is subjected to a forming operation,during which the initially insulating properties of the resistiveswitching layer are altered and the ReRAM cell is configured into theinitial LRS or HRS. The forming operation may include a very short highdischarge current peak associated with a forming voltage, which is usedto set the LRS level of the resistive switching layer for subsequentswitching as outlined above. In this case, a resistive switching layerwith very low levels (e.g., 100-30 kOhm) of resistance in the LRS may belimited in terms of scaling down. This difficulty may be resolved bypositioning such resistive switching layers in series with othercomponents providing additional resistance to the overall ReRAM cell.

Processing Examples

FIG. 3A illustrates a process flowchart corresponding to method 300 offorming a ReRAM cell, in accordance with some embodiments. Method 300may commence with providing a substrate during operation 302. Thesubstrate is used for receiving various deposited components of theReRAM cell. Furthermore, the same substrate often is used for receivingcomponents of multiple ReRAM cells. For example, large memory cellarrays may be formed on the same substrate. Components of multiple ReRAMcells may be formed from the same set of initial layers formed on thatsubstrate. The substrate provided in operation 302 may include one ormore signal lines or contacts. These lines or contacts provide anelectrical connection to a bottom electrode. In some embodiments, thebottom electrode formed in operation 302 serves as a signal line. In asimilar manner, a top electrode formed in operation 310 may function asa signal line or it may be connected to a separate signal line.

Method 300 may proceed with forming a bottom electrode during operation304. The bottom electrode may be formed using ALD, CVD, sputtering, orsome other techniques. For example, a titanium nitride electrode may beformed using sputtering. Deposition of the titanium nitride electrodemay be performed using a titanium target in a nitrogen atmospheremaintained at a pressure of between about 1-20 mTorr. The power may bemaintained at 150-500 Watts that may result in a deposition rate ofabout 0.5-5 Angstroms per second (depending on the size of the targetsample and other process parameters). Some of the provided processparameters are for illustrative purposes only and generally depend ondeposited materials, tools, deposition rates, and other factors.

Method 300 may proceed with forming a base layer that includes hafniumand aluminum oxides or nitrides in operation 306. The base layercontaining hafnium and aluminum oxides may be then nitridized duringoperation 308 to form a resistive switching layer that also includesnitrogen. In a similar manner, the base layer containing hafnium andaluminum nitrides may be then oxidized during operation 308 to form aresistive switching layer that also includes sufficient amounts ofoxygen. In some embodiments, operations 306 and 308 are combined suchthat the initially formed base layer already includes sufficient amountsof nitrogen and oxygen and further nitriding or oxidation operations arenot needed.

The base layer (or the final resistive switching layer) may be formedusing reactive sputtering, ALD, or other techniques. For example, thebase layer may be formed using reactive sputtering by employing ahafnium target and an aluminum target in a 20-60% oxygen atmosphere.Power of 100-1000 Watts (W) may be used to achieve deposition rates ofbetween about 0.1 and 1.0 Angstroms per second. These process parametersare provided as examples and generally depend on deposited materials,tools, deposition rates, and other factors. Nitrogen may be added intothis deposition environment to be incorporated directly into the baselayer.

In some embodiments, the base layer is formed using ALD. This techniqueincludes one or more cycles, each involving the following four steps:introducing one or more precursors into the depositing chamber to forman absorbed layer, followed by purging these precursors reactive agents,and then introducing reactive agents that will react with the absorbedlayer to form a portion of or the entire base layer, followed by purgingthe reactive agents reactive agents. Selection of precursors andprocessing conditions depend on desired composition, morphology, andstructure of each portion of the electrode.

A layer formed during each atomic layer deposition cycle described abovemay be between about 0.25 and 2 Angstroms thick. The cycle may berepeated multiple times until the overall base layer (and subsequentlythe thickness of the resistive switching layer) reaches it desiredthickness. In some embodiments, the thickness of the resistive switchinglayer is less than 50 Angstroms or, more specifically, less than 30Angstroms. In general, the thickness of the resistive switching layer isat least about 10 Angstroms or, more specifically, at least about 20Angstroms, as thin films may be considered too “leaky.” In someembodiments, ALD cycles are repeated using different precursors. Assuch, different portions of the same base layer may have differentcompositions as will be later described with reference to FIG. 3B.

ALD techniques are now briefly described to provide better understandingof various processing features. A hafnium containing precursor or analuminum containing precursor is introduced into the ALD chamber andallowed to flow over the deposition surface (which may have previouslydeposited ALD layers) provided therein. The one or more precursors areintroduced in the form of pulses. Between the pulses, the reactionchamber is purged, for example, with an inert gas to remove unreactedprecursors, reaction products, and other undesirable components from thechamber.

The introduced precursor adsorbs (e.g., saturatively chemisorbs) on thedeposition surface. Subsequent pulsing with a purging gas removes excessprecursor from the deposition chamber. In some embodiments, purging isperformed before full saturation of the substrate surface occurs withthe precursors. In other words, additional precursor molecules couldhave been further adsorbed on the substrate surface if the purging wasnot initiated so early. Without being restricted to any particulartheory, it is believed that partial saturation can be used to introducedefects into the formed layer, e.g., during forming of a resistiveswitching layer.

After the initial precursor pulsing and purging of one or more metalcontaining precursors, a subsequent pulse introduces a reactant agent.The reactant agent reacts with the adsorbed metal containing moleculesto form oxides and/or nitrides. Reaction byproducts and excess reactantsare purged from the deposition chamber. The saturation during thereaction and purging stages makes the growth self-limiting. This featurehelps to improve deposition uniformity and conformality and allows moreprecise control of the resulting resistive switching characteristics.

The temperature of the substrate during atomic layer deposition may bebetween about 200° C. to 350° C. The precursor may be either in gaseousphase, liquid phase, or solid phase. If a liquid or solid precursor isused, then it may be transported into the chamber an inert carrier gas,such as helium or nitrogen.

Some examples of hafnium containing precursors includebis(tert-butylcyclopentadienyl)dimethyl hafnium (C₂₀H₃₂Hf),bis(methyl-η5-cyclopentadienyl) methoxymethyl hafnium(HfCH₃(OCH₃)[C₅H₄(CH₃)]₂), bis(trimethylsilyl)amido hafnium chloride([[(CH₃)₃Si]₂N]₂HfCl₂), dimethylbis(cyclopentadienyl)hafnium((C₅H₅)₂Hf(CH₃)₂), hafnium isopropoxide isopropanol adduct (C₁₂H₂₈HfO₄),tetrakis(diethylamido)hafnium ([(CH₂CH₃)₂N]₄Hf)—also known as TEMAH,tetrakis(ethylmethylamido)hafnium ([(CH₃)(C₂H₅)N]₄Hf),tetrakis(dimethylamido) hafnium ([(CH₃)₂N]₄Hf)—also known as TDMAH, andhafnium tert-butoxide (HTB). Some hafnium containing precursors can berepresented with a formula (RR′N) 4Hf, where R and R′ are independenthydrogen or alkyl groups and may be the same or different. Some examplesof aluminum containing precursors include aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate)(Al(OCC(CH₃)₃CHCOC(CH₃)₃)₃), triisobutyl aluminum ([(CH₃)₂CHCH₂]₃Al),trimethyl aluminum ((CH₃)₃Al)—also known as TMA, Tris(dimethyl amido)aluminum (Al(N(CH₃)₂)₃). The nitrogen containing oxidizing agent mayinclude ammonia (NH₃), which in some embodiments may be mixed withcarbon monoxide (CO). Some examples of suitable oxidizing agentscontaining oxygen include water (H2O), peroxides (organic and inorganic,including hydrogen peroxide H₂O₂), oxygen (O₂), ozone (O₃), oxides ofnitrogen (NO, N₂O, NO₂, N₂O₅), alcohols (e.g., ROH, where R is a methyl,ethyl, propyl, isopropyl, butyl, secondary butyl, or tertiary butylgroup, or other suitable alkyl group), carboxylic acids (RCOOH, where Ris any suitable alkyl group as above), and radical oxygen compounds(eg., O, O₂, O₃, and OH radicals produced by heat, hot-wires, and/orplasma).

Different examples of operation 306 will now be explained with referenceto FIG. 3B. Operation 306 may start with pulsing of a first metalcontaining precursor into the ALD chamber during sub-operation 322. Thefirst metal containing precursor may include hafnium or aluminum. Forexample, the hafnium containing precursor is introduced to the chamberas a pulse. A purge gas may be provided continuously with the pulse ormay be discontinued during the pulse. The purge gas is non-reactive orinert at given process conditions and may include nitrogen (N₂) orhelium (He). At least a portion of the precursor adsorbs onto or reactswith the surface of the bottom electrode. Adsorption of the precursordepends on the availability of adsorption sites. When these sites areall consumed (i.e., a fully saturated processing layer is formed), nomore metal containing precursor can adsorb, and any remaining precursoris removed by flowing the purge gas.

Once the first metal containing precursor is adsorbed on the depositionsurface and remaining portions of the precursor are purged from thechamber, operation 306 may proceed with reaction of the adsorbedprecursor during optional sub-operation 324. In this sub-operation, apulse of a reactant agent is provided to the deposition chamber. Thereactant agent reacts with the metal containing precursor remaining onthe substrate and forms a metal oxide film or a metal nitride firm. Thereactant agent is then purged from the deposition chamber. This cyclemay be repeated until the desired thickness of first metal oxide (ornitride) is formed as reflected by decision block 326. In someembodiments, reactant agents are changed from one cycle to another toform both metal oxide films and metal nitride films in the stack. Thedistribution of nitrogen in the stack may be uneven as further describedbelow with reference to FIG. 4. For example, most of the nitrogen may beconcentrated near one surface of the stack. As such, initial or finalreaction sub-operations may include more reaction operations involvingnitrogen containing reactant agents than other reaction sub-operations.

The process then continues with pulsing of a second metal precursor intothe chamber during sub-operation 328. This precursor contains adifferent metal than the first metal containing precursor used insub-operation 322. For example, if a hafnium containing precursor wasused in sub-operation 322, then an aluminum containing precursor will beused in used in sub-operation 328. The second metal containing precursoris then oxidized to form a metal oxide containing the second metalduring sub-operation 330. This cycle may be repeated until the desiredthickness of a built up stack is formed as reflected by decision block332. A number of cycles depositing the first metal oxide and a number ofcycles depositing the second metal oxide are controlled to achieve aspecific composition of the base layer. The following table providesexperimental results indicating different concentrations of metals inthe base layers caused by process variations.

TABLE Number of Hf layers:Number Concentration of Hf/(Hf + Al) in a ofAl layers resulting nano-laminate layer, atomic % 1:1 ~30-40% 1:3~20-30% 1:5 ~10-20%

The overall process of depositing oxides and/or nitrides of the firstand second metals may be repeated a number of times until the overallbase layer reach a predetermined thickness as reflected by the decisionblock 334. This approach is referred to as nanolamination ALD, in whichmultiple layers of different metal oxides form a stack.

In an alternative approach, which is referred to as staggered pulse ALD,sub-operation 324 is skipped and process continues with pulsing of asecond metal containing precursor into the ALD chamber during operation328. In this approach, the two metal containing precursors are adsorbedon the deposition surface prior to reaction. A reactant agent is thenintroduced into the chamber during operation 330 to convert bothprecursors into two corresponding metal nitrides or oxides. The order ofadsorption and reactivity of the two precursors determine thecomposition of the resulting base layer. For example, if TMA isintroduced into the chamber prior to TDMAHf, then resulting base layercontains predominantly aluminum oxide with very little hafnium present.However, if TMAHf is introduced into the chamber prior to TDMA, thenresulting base layer contains about 70 atomic percent of hafniumrelative to the total metal amount in the resulting layer. In someembodiments, both nanolamination ALD and staggered pulse ALD approachesmay be used to form the same base layer.

Returning to FIG. 3A, after forming the base layer during operation 306,method 300 may proceed with forming a resistive switching layer from thebase layer during optional operation 308. In some embodiments, the baselayer already includes all required nitrogen and operation 308 isskipped. Otherwise, operation 308 may be used to introduce nitrogen intothe base layer to convert this layer into a resistive switching layer.It should be noted that the resistive switching layer may be furtherprocessed to create initial conductive filaments by applying a formationvoltage. Furthermore, the resistive switching layer may be subjected toannealing to more distribute evenly distribute material in the resistiveswitching layer, e.g., at 750° C. for 1 minutes in a forming gas.

Operation 308 may involve various nitridation techniques such as plasmanitridation (e.g., de-coupled plasma nitridation) and thermalnitridation (e.g., anneal). In plasma nitridation, the reactivity of thenitriding media is due to the ionized state of the gas. Electric fieldsare used to generate ionized species of the gas (e.g., molecularnitrogen) around the processed surface. The ionized species react withmaterials in the base layer, which cause introduction of nitrogen intothe layer. Thermal nitridation may use a nitrogen rich reactive gas,such as ammonia (NH₃). When the gas comes into contact with the surfaceof the heated base layer, the gas disassociates releasing nitrogen (andhydrogen if ammonia is used). This nitrogen then diffuses into thelayer. These processes may be specifically controlled to achievespecific concentrations and distributions of nitrogen in the resistiveswitching layer. Plasma nitridation may used in some embodiments. Insome embodiments, various metal nitrides deposited using ALD techniquescan be converted into oxynitrides (partially oxidized) after beingannealed for 1 min in oxygen containing environment at high temperatures(e.g., 400° C.-700° C.). At 750° C., the oxygen anneal may completeconvert the nitride into oxide and therefore, the temperature or oxygenconcentration should be limited.

Once the resistive switching layer is formed, the process may proceedwith forming of a top electrode during operation 310. The top electrodemay also be deposited in a manner similar to the bottom electrode asdescribed above with reference to operation 304. Other layers, such asinterface or capping layers, current limiting layers, and other layermay be deposited in the stack, e.g., between the top electrode and theresistive switching layer and/or between the bottom electrode and theresistive switching layer.

ReRAM Cell Examples

FIG. 4 illustrates a schematic representation of resistive switchingReRAM cell 400, in accordance with some embodiments. Resistive switchingReRAM cell 400 includes substrate 402, which may include a signal line.Alternatively, bottom electrode 404 may serve as a signal line.Substrate 402 provides a surface for deposition of bottom electrode 404.Bottom electrode 404 is disposed between substrate 402 and resistiveswitching layer 406. Top electrode 408 is provided above resistiveswitching layer 406.

Resistive switching layer 406 includes hafnium, aluminum, oxygen, andnitrogen. It should be noted that resistive switching layer 406 mayinclude any concentrations of these four elements and are not limited byany stoichiometric ratios. In some embodiments, the concentration ofnitrogen in the switching layer is between about 1 and 20 atomic percentor, more specifically, between about 2 and 5 atomic percent. Theconcentration of nitrogen in the resistive switching layer may be atleast three times less than the concentration of oxygen. In someembodiments, the concentration of oxygen in the resistive switchinglayer is between about 30 and 60 atomic percent or, more specificallybetween about 40 and 50 atomic percent.

In some embodiments, nitrogen is unevenly distributed within resistiveswitching layer 406. For example, more nitrogen may be present at theinterface with top electrode 408 or bottom electrode 404. Specifically,resistive switching layer 406 may include a sub-layer at the interfacewith one of these electrodes such that more than 50 atomic percent ormore than 75 atomic percent or even more than 90 atomic percent of allnitrogen present in resistive switching layer 406 is provided withinthis sublayer. In some embodiments, the thickness of this sublayer isone tenth of the thickness of resistive switching layer 406.

In some embodiments, the concentration of hafnium in the resistiveswitching layer is between about 5 and 40 atomic percent or, morespecifically, between about 10 and 30 atomic percent. The concentrationof aluminum in the resistive switching layer is between 3 and 20 atomicpercent or, more specifically, between about 5 and 10 atomic percent. Insome embodiments, the concentration of hafnium in the resistiveswitching layer is at least twice greater than the concentration ofaluminum.

The material of resistive switching layer 406 is substantially amorphousafter formation of this layer. In some embodiments, resistive switchinglayer 406 remains substantially amorphous after further processing ofthe layer, such as annealing, applying a formation voltage, and otheroperations. Furthermore, in some embodiments, resistive switching layer406 remains substantially amorphous during operation of ReRAM cell,i.e., applying switching voltages and reading voltages that drivecorresponding currents.

In some embodiments, the thickness of restive switching layer 406 isbetween about 20 and 100 Angstroms or, more specifically, between about40 and 70 Angstroms, for example, about 50 Angstroms. The thickness oftop and bottom electrodes 404 and 408 may be at least about 30 and 1000Angstroms or, more specifically, between about 100 and 500 Angstroms. Insome embodiments, the thickness of one or both electrodes is less than50 Angstroms. Such electrodes may be deposited using ALD techniques.

Electrodes 404 and 408 provide electronic communication to resistiveswitching layer 406 of ReRAM cell 400. One or both electrodes maydirectly interface resistive switching layer 406 or be spaced apart byother layers, such as barrier layers, current limiting layer, and thelike. Depending on the materials used for electrode construction, theelectrode (e.g., an electrode formed from titanium nitride) itself mayalso serve as an adhesion layer and/or barrier layer. In someembodiments, one or both electrodes are also function as signal lines(i.e., bit and/or word lines) and are shared by other ReRAM cells.

Some examples of electrode materials include silicon (e.g., n-dopedpoly-silicon and p-doped poly-silicon), silicides, silicide-germanides,germanides, titanium, titanium nitride (TiN), platinum, iridium, iridiumoxide, ruthenium, ruthenium oxide, and the like. Generally, anysufficiently conductive material may be used to form an electrode. Insome embodiments, barrier layers, adhesion layers, antireflectioncoatings and/or the like may be used with the electrodes and to improvedevice performance and/or aid in device fabrication.

In some embodiments, one electrode may be a higher work functionmaterial, and the other electrode may be a lower work function materialthan the resistive switching layer. For example, a noble or near noblemetal (i.e., a metal with a low absolute value free energy change (|ΔG|)of oxide formation) may be used for one electrode. Specific examplesinclude iridium, iridium oxide, platinum, ruthenium, and rutheniumoxide. The other electrode may be a lower work function material, suchas titanium nitride. In some embodiments, the reset pulse at theelectrode having the higher work function may be a positive pulse.

In some embodiments, one or both electrodes may be multi-layerelectrodes formed by one or more different materials. For example, anelectrode can include a base layer and capping layer. The base layer mayinclude ruthenium, ruthenium oxide, iridium, iridium oxide, platinum,and various combinations thereof. The capping layer may includetungsten, tungsten carbonitride, and/or tungsten carbon. The multi-layerelectrodes can be used to improve adhesion properties and performance ofmemory elements in some configurations and embodiments.

ReRAM cell 400 may include another layer (not shown) that is operable asa current limiting layer. A material for this layer may have a suitablework function for controlling the electron flow through ReRAM cell. Thisspecific selection may alter the magnitude of the generated switchingcurrents. In some embodiments, the current limiting layer is used toincrease or decrease the formed barrier height at the interface withresistive switching layer 406. This feature is used to improve currentflowing characteristics and reduce the magnitude of the switchingcurrents. It should be noted that these changes in the barrier heightwill generally not affect the current ratio (I_(ON)/I_(OFF)), and thusnot impacts detectability of different resistive states.

In some embodiments, the current limiting layer is between about 50Angstroms and 1000 Angstroms thick, such as between about 200 Angstromsand 50 Angstroms. This layer may be formed from a material that has aresistivity of between about 5 Ohm-cm and 500 Ohm-cm, such as betweenabout 50 Ohm-cm and 150 Ohm-cm. In some embodiments, the currentlimiting layer is formed such that its resistance (R_(RL)) is betweenabout 10 kilo-Ohm and about 10 mega-Ohm, such as between about 100kilo-Ohm and about 1 mega-Ohm.

Resistivity is an intrinsic property of the material and can becontrolled by adjusting the composition of the material. Some specificexamples, include adding alloying elements or doping atoms and/oradjusting the morphological structure of the materials, (e.g., shiftingfrom amorphous to crystal structure). In some embodiments, a currentlimiting layer may include titanium oxide doped with niobium, tin oxidedoped with antimony, or zinc oxide doped with aluminum. Theconcentration of dopant materials in the base material may be betweenabout 0.5 and 25 atomic % or, more specifically, between about 1 and 10atomic %.

Other examples of materials suitable for the current limiting layerinclude titanium nitride (Ti_(x)N_(y)), tantalum nitride (Ta_(x)N_(y)),silicon nitride (SixNy), hafnium nitride (Hf_(x)N_(y)) or titaniumaluminum nitride (Ti_(x)Al_(y)N_(z)) layer. Such layers may be formedusing an ALD, CVD or PVD process as further described below.

Apparatus Examples

FIG. 5 illustrates a schematic representation of atomic layer depositionapparatus 500 for fabricating ReRAM cells, in accordance with someembodiments. For clarity, some components of apparatus 500 are notincluded in this figure, such as a wafer-loading port, wafer lift pins,and electrical feed throughs. Apparatus 500 includes deposition chamber502 connected to processing gas delivery lines 504. While FIG. 5illustrates three delivery lines 504, any number of delivery lines maybe used. Each line may be equipped with a valve and/or mass flowcontroller 506 for controlling the delivery rates of processing gasesinto deposition chamber 502. In some embodiments, gases are providedinto delivery port 508 prior to exposing substrate 510 to processinggases. Deliver port 508 may be used for premixing gases (e.g.,precursors and diluents) and even distribution of gases over the surfaceof substrate 510. Delivery port 508 is sometimes referred to as ashowerhead. Delivery port 508 may include a diffusion plate 509 havingwith multiple holes for gas distribution.

Deposition chamber 502 encloses substrate support 512 for holdingsubstrate 510 during its processing. Substrate support 512 may be madefrom a thermally conducting metal (e.g., W, Mo, Al, Ni) or other likematerials (e.g., a conductive ceramic) and may be used to maintain thesubstrate temperature at desired levels. Substrate support 512 may beconnected to drive 514 for moving substrate 510 during loading,unloading, process set up, and sometimes even during processing.Deposition chamber 502 may be connected to vacuum pump 516 forevacuating reaction products and unreacted gases from deposition chamber502 and for maintaining the desirable pressure inside chamber 502.

Apparatus 500 may include system controller 520 for controlling processconditions during electrode and resistive switching layer deposition andother processes. Controller 520 may include one or more memory devicesand one or more processors with a CPU or computer, analog and/or digitalinput/output connections, stepper motor controller boards, etc. In someembodiments, controller 520 executes system control software includingsets of instructions for controlling timing, gas flows, chamberpressure, chamber temperature, substrate temperature, RF power levels(if RF components are used, e.g., for process gas dissociation), andother parameters. Other computer programs and instruction stored onmemory devices associated with controller may be employed in someembodiments.

Memory Array Examples

A brief description of memory arrays will now be described withreference to FIGS. 6A and 6B to provide better understanding to variousaspects of thermally isolating structures provided adjacent to ReRAMcells and, in some examples, surrounding the ReRAM cells. ReRAM cellsdescribed above may be used in memory devices or larger integratedcircuits (IC) that may take a form of arrays. FIG. 6A illustrates amemory array 600 including nine ReRAM cells 602, in accordance with someembodiments. In general, any number of ReRAM cells may be arranged intoone array. Connections to each ReRAM cell 602 are provided by signallines 604 and 606, which may be arranged orthogonally to each other.ReRAM cells 602 are positioned at crossings of signal lines 604 and 606that typically define boundaries of each ReRAM cell in array 600.

Signal lines 604 and 606 are sometimes referred to as word lines and bitlines. These lines are used to read and write data into each ReRAM cell602 of array 600 by individually connecting ReRAM cells to read andwrite controllers. Individual ReRAM cells 602 or groups of ReRAM cells602 can be addressed by using appropriate sets of signal lines 604 and606. Each ReRAM cell 602 typically includes multiple layers, such as topand bottom electrodes, resistive switching layer, embedded resistors,embedded current steering elements, and the like, some of which arefurther described elsewhere in this document. In some embodiments, aReRAM cell includes multiple resistive switching layers provided inbetween a crossing pair of signal lines 604 and 606.

As stated above, various read and write controllers may be used tocontrol operations of ReRAM cells 602. A suitable controller isconnected to ReRAM cells 602 by signal lines 604 and 606 and may be apart of the same memory device and circuitry. In some embodiments, aread and write controller is a separate memory device capable ofcontrolling multiple memory devices each one containing an array ofReRAM cells. Any suitable read and write controller and array layoutscheme may be used to construct a memory device from multiple ReRAMcells. In some embodiments, other electrical components may beassociated with the overall array 600 or each ReRAM cell 602. Forexample, to avoid the parasitic-path-problem, i.e., signal bypasses byReRAM cells in their low resistance state (LRS), serial elements with aparticular non-linearity must be added at each node or, morespecifically, into each element. Depending on the switching scheme ofthe ReRAM cell, these elements can be diodes or varistor-type elementswith a specific degree of non-linearity. In the same other embodiments,an array is organized as an active matrix, in which a transistor ispositioned at each node or, more specifically, embedded into each cellto decouple the cell if it is not addressed. This approach significantlyreduces crosstalk in the matrix of the memory device.

In some embodiments, a memory device may include multiple array layersas, for example, illustrated in FIG. 6B. In this example, five sets ofsignal lines 614 a-b and 616 a-c are shared by four ReRAM arrays 612a-c. As with the previous example, each ReRAM array is supported by twosets of signal lines, e.g., array 612 a is supported by 614 a and 616 a.However, middle signal lines 614 a-b and 616 b, each is shared by twosets ReRAM arrays. For example, signal line set 614 a providesconnections to arrays 612 a and 612 b. Top and bottom sets of signallines 616 a and 616 c are only used for making electrical connections toone array. This 3-D arrangement of the memory device should bedistinguished from various 3-D arrangements in each individual ReRAMcell.

CONCLUSION

Although the foregoing examples have been described in some detail forpurposes of clarity of understanding, the invention is not limited tothe details provided. There are many alternative ways of implementingthe invention. The disclosed examples are illustrative and notrestrictive.

1. A resistive random access memory cell comprising: a first layeroperable as a first electrode; a second layer operable as a secondelectrode; and a third layer operable as a resistive switching layer anddisposed between the first layer and the second layer, the third layercomprising hafnium, aluminum, oxygen, and nitrogen, wherein a thicknessof the third layer is between 20 and 100 Angstroms.
 2. The resistiverandom access memory cell of claim 1, wherein a concentration of oxygenin the third layer is between 30 and 60 atomic percent.
 3. The resistiverandom access memory cell of claim 1, wherein a concentration ofnitrogen in the third layer is between 1 and 20 atomic percent.
 4. Theresistive random access memory cell of claim 1, wherein a concentrationof nitrogen in the third layer is at least three times less than aconcentration of oxygen.
 5. The resistive random access memory cell ofclaim 1, wherein a concentration of hafnium in the third layer is atleast twice greater than a concentration of aluminum.
 6. The resistiverandom access memory cell of claim 1, wherein a concentration of hafniumin the third layer is between 5 and 40 atomic percent.
 7. The resistiverandom access memory cell of claim 1, wherein a concentration ofaluminum in the third layer is between 3 and 20 atomic percent. 8.(canceled)
 9. The resistive random access memory cell of claim 1,wherein the third layer is substantially amorphous.
 10. The resistiverandom access memory cell of claim 1, wherein the nitrogen is unevenlydistributed within the third layer.
 11. (canceled)
 12. The resistiverandom access memory cell of claim 11, wherein the first layer is formedafter the third layer.
 13. The resistive random access memory cell ofclaim 12, wherein the first layer directly interfaces the third layer.14. The resistive random access memory cell of claim 13, wherein thefirst layer comprises one of tantalum nitride, titanium nitride, ortungsten nitride.
 15. The resistive random access memory cell of claim1, wherein the first layer or the second layer has a thickness of lessthan 50 Angstroms. 16-20. (canceled)
 21. The resistive random accessmemory cell of claim 1, wherein a portion of the third layer comprisesmore than 50 atomic percent of all nitrogen presented in the thirdlayer, and wherein a thickness of the portion is one tenth of athickness of the third layer.
 22. The resistive random access memorycell of claim 21, wherein the portion of the third layer comprises morethan 75 atomic percent of all nitrogen presented in the third layer. 23.The resistive random access memory cell of claim 21, wherein the portionof the third layer comprises more than 90 atomic percent of all nitrogenpresented in the third layer.
 24. The resistive random access memorycell of claim 21, wherein the portion of the third layer interfaces withthe first layer.
 25. The resistive random access memory cell of claim13, wherein the first layer comprises titanium nitride.
 26. Theresistive random access memory cell of claim 25, wherein the secondlayer comprises doped polysilicon.
 27. The resistive random accessmemory cell of claim 1, wherein the first electrode comprises a firstportion comprising one of ruthenium, ruthenium oxide, iridium, iridiumoxide, or platinum and a second portion comprising one of tungsten,tungsten carbonitride, or tungsten carbon.